As old-school cocktails like the Manhattan and Sazerac return to bar menus, bitters—the liquor used to give these drinks their aromatic essence—have seen a surge in popularity as well. In the late 19th century, cocktail aficionados added the high-proof grain alcohol, steeped in spices and bitter herbs (like gentian and wormwood), for its medicinal properties. Today, mixologists are making their own bitters to infuse their sips with unexpected flavors.

The Bitter Bar
This Boulder bar lives up to its name: It makes grapefruit-coriander, Asian five-spice, grapefruit, and lemon-rose petal bitters. But the rare, French-style Amer Picon is the bar’s favorite. Its orange notes give the Brooklyn—a cocktail of rye whiskey and dry vermouth— a delicate elegance. 835 Walnut St., Boulder, 303-442-3050,

Frasca Food & Wine
This summer, Frasca’s bar manager, Bryan Dayton, set out to make a version of the 19th-century Boker’s bitters. As in the original, he added cardamom and orange peel to 90-proof whiskey—but then he threw in star anise and cloves. He combines the resulting, softly flavored bitters with ginger, vanilla, and artichoke liquors in the Cascade Sunset for a cocktail that hints of sweetness and autumn. 1738 Pearl St., Boulder, 303-442-6966,

After a round of gin and tonics with homemade tonic at the New York City restaurant Per Se, Mizuna co-owner Jacqueline Bonanno decided she, too, wanted to make her own tonics and bitters. She searched her German grandmothers’ recipes and found the blend of gentian, cassia, orange peel, and Everclear that now defines Mizuna’s Manhattan. 225 E. Seventh Ave., 303-832-4778,

Instead of following the traditional bitters-making technique (saturating spices in alcohol for 10-plus days), TAG bar manager Mike Henderson vacuum-seals herbs and slowly cooks them in a water bath. After two days, he adds the mixture of elderflower, orange, lemon grass, and lavender to Everclear. Eventually, he stirs that concoction into drinks such as An Honest Mistake (gin, absinthe, sugar, and bitters). 1441 Larimer St., 303-996-9985,